Deciphering an odd floppy disk format.

Fred Cisin cisin at
Mon Feb 15 13:51:18 CST 2021

On Mon, 15 Feb 2021, Mattis Lind via cctalk wrote:
> My guess is that the data that follows the sector ID is some kind of
> checksum.

yes.  well, sorta. 16 bit CRC

A typical IBM/WD style format has:

a gap
Index Address Mark
a gap   (note that WD can use a shorter post index gap than NEC can)

and then, repeated for each sector:
a gap
ID Address Mark, 
C Cylinder number
H Head number    (0,1)
R Sector number
N Sector Length  (0:128, 1:256, 2:512, 3:1024)
16 bit CRC of the sector header
a gap
Data Address Mark
Sector data  (128, 256, 512, or 1024 bytes.  Larger is possible, but unlikely)
16 bit CRC of the data
a gap

(This is from memory (error prone?).  The WD1791 datasheet should have 
more detail, including CRC algorithm?, the specific requirements for the 
address marks, and gap contents (write splice, synchronization, etc.))

Grumpy Ol' Fred     		cisin at

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